This invention generally relates to optical communications, and in particular to a method and system for demodulating an optical carrier in an optical communications network.
The backbone of point-to-point information transmission networks is a system of optically amplified dense wavelength division multiplex (DWDM) optical links. Due to the increase in bit rate applied to each wavelength of a multiplexed signal, and the simultaneous increase in the number of channels, the finite width of the erbium gain window of conventional erbium-doped optical amplifiers (EDFAs) could become a significant obstacle to further increases in capacity. Conventional EDFAs have a 35 nm gain bandwidth which corresponds to a spectral width of 4.4 THz. System demonstrations of several Tbit/s are already a reality and the spectral efficiency, characterized by the value of bit/s/Hz transmitted, is becoming an important consideration. Currently, high-speed optical transmission employs binary amplitude keying, using either non-return-to-zero (NRZ) or return-to-zero (RZ) signaling formats, in which data is transmitted in the form of optical pulses having a single symbol level.
One technique which has been proposed which allows an improvement of spectral efficiency is the use of quaternary phase shift keying (QPSK) [S. Yamazaki and K. Emura, (1990) “Feasibility study on QPSK optical heterodyne detection system”, J. Lightwave Technol., vol. 8, pp. 1646-1653]. In optical QPSK the phase of light generated by a transmitter laser is modulated either using a single phase modulator (PM) driven by a four-level electrical signal to generate phase shifts of 0, π/2, π or 3π/2 representative of the four data states, or using two concatenated phase modulators which generate phase shifts of 0 or π/2 and π or 3π/2 respectively. A particular disadvantage of QPSK is that demodulation requires, at the demodulator, a local laser which is optically phase-locked to the transmitter laser. Typically this requires a carrier phase recovery system. For a WDM system a phase-locked laser will be required for each wavelength channel. It further requires adaptive polarization control which, in conjunction with a phase recovery system, represents a very high degree of complexity. Furthermore, systems that require a coherent local laser are sensitive to cross-phase modulation (XPM) in the optical fiber induced by the optical Kerr non-linearity, which severely restricts the application to high capacity DWDM transmission.
It has also been proposed to use differential binary phase shift keying (DBPSK) [M. Rohde et al (2000) “Robustness of DPSK direct detection transmission format in standard fiber WDM systems”, Electron. Lett., vol. 36]. In DBPSK, data is encoded in the form of phase transitions of 0 or π, in which the phase value depends upon the phase of the carrier during the preceding symbol interval. A Mach-Zehnder interferometer with a delay in one optical path equal to the symbol period is conventionally used to demodulate the optical signal. Although DBPSK does not require a phase-locked laser at the receiver, it does not provide any significant advantages compared to conventional amplitude NRZ signaling.
Differential quadrature phase shift keying (DQPSK) is a popular format for upgrading installed links to 40 Gb/s, due to its spectral efficiency and tolerance to chromatic and polarization mode dispersion. However it has been shown that there can be significant penalties as a result of XPM when DQPSK channels are transmitted alongside 10 Gb/s on-off-keyed (OOK) channels.
XPM causes a penalty in DQPSK channels because of random variations in the intensity of the OOK channels (due to the random nature of the data). This results in phase noise on the DQPSK channels which is translated to intensity noise and therefore errors when the signal is demodulated at the receiver.
The XPM penalty can be reduced by using both a high local and average dispersion or narrow filtering; however, when upgrading installed systems both the filtering and the dispersion are generally fixed before the upgrade takes place and so cannot be changed without interrupting service. The XPM penalty can also be reduced by controlling power of both the DQPSK and the OOK channels; however, in the case of the OOK channels this has been previously set to give the required performance and so cannot necessarily be easily changed while maintaining the performance of those channels.